Filter elements having a preferably folded, and thus pleated filter medium, are used in systems and facilities, in which fluids are used as operating media, to ensure the proper condition of the particular fluids. The fluids in question can be, for example, lubricating oils, fuels, and hydraulic liquids, process water, and air flows, which are subject to contaminants, or which contain contaminants present as colloids or as solid particles. In particular, in the case of higher-value facilities, for safety reasons and for economic reasons the filter elements in use need to deliver full efficiency beyond the provided usage times. Malfunctions of the filter elements would cause operational disruptions, even system failures, which can result in economic damage.
During the operation of filter elements having star-folded filter media, one possible cause of a reduced filter function can be seen in that during the flow through the folded structure of the filter medium, the folded structure is subjected to local stresses. Due to local stresses, local blockage formations of the folds can occur, whereby isolated areas of the filter surface come into direct contact with one another. At these regions, no through flow or only a slight through flow then takes place in relation to the unblocked filter folds. The reduced flow is accompanied by a correspondingly reduced dirt absorption.
One proposed solution to remedy this error source, which is disclosed in DE 10 2004 054 245 A1, provides stabilization in the form of band-shaped surface elements in a filter element of the aforementioned type. The surface elements are laid as a closed ring around the outer side of the pleated filter medium. Connecting points or connecting zones to the adjoining fold backs of the filter medium are fixed on the inner sides of the bands. The connecting points are each formed by spot welds, at which the materials of the outer layer of the filter medium and of the adjoining band on its inner side are fixedly fused with one another by an energy input method, for example, in the form of an ultrasound or laser welding method.
EP 2 559 467 A1 criticized that prior art since the large number of pleated folds requires carrying out a plurality of welding operations, making the production more complicated and costly. Furthermore, the freedom of the material selection is restricted insofar as a material pair must be selected for the band-shaped surface elements and the outer layer of the filter medium, which is compatible for an ultrasound or laser welding operation to be carried out directly thereon.
To counter these disadvantages, the solution according to EP 2 559 467 A1 proposes connection in the form of an adhesive application. In cooperation with the fold backs of the filter medium facing toward the respective support jacket, the adhesive forms connecting zones. The connecting zones are distributed in such a way that all circumferential fold backs, which face toward the respective protective jacket, are located inside at least one connecting zone. Anchoring points are then formed for all fold backs on the facing support jacket, which effectively stabilizes the fold geometry against acting flow forces. The adhesive application is provided on the support jacket or on the filter medium itself. Adhesive applications in the form of strip-shaped adhesive beads or adhesive clusters are preferably used.
In these prior art solutions, no fluid passage point is provided in the region of the adhesive application points or in the region of the welded-on closed fixing bands, which would enable fluid to flow through the filter element in this respective region. In this regard, the filtration performance is also impaired in these known solutions because of the fluid impermeability of the support for the fold fixation.